(1) Field of the Invention
The present invention relates to pressure control techniques which control the pressure in a low pressure processing chamber such as a plasma processing chamber and more particularly to a pressure control technique which enables high speed control regardless of plasma dissociation or change in the effective flow rate.
(2) Description of the Related Art
FIG. 2 illustrates a device which controls the pressure in a low pressure processing chamber. As shown in FIG. 2, a throttle valve 3 is located between a low pressure processing chamber 1 and an exhaust device 2. In this control device, the reading of a pressure gauge 4 connected with the low pressure processing chamber 1 through an arithmetic and control unit 13 is fed back and reflected in the opening degree of the throttle valve 3 so that the pressure in the low pressure processing chamber is automatically controlled (see Japanese Patent Application Laid-Open Publication No. H10-11152).
There are various known methods of feeding back the reading of a pressure gauge for the valve opening degree and the most commonly used method is PID control. Usually in a typical PID control method, valve opening operation amount ΔVV is calculated for each control cycle in accordance with a PID control calculation formula (Formula 1 below) to control the opening degree of the valve.
[Formula 1]ΔVV 32 VVn+1−VVn=Gi(Pn−P0)+Gp(Pn−Pn−1)+Gd(Pn−2Pn−1+Pn−2)   (1)Here,    ΔVV: Valve opening operation amount (%)    VVn+1: Next valve opening operation degree (%)    VVn: Current valve opening degree (%)    Pn: Current pressure (Pa)    Pn−1: Previous pressure (Pa)    Pn−2: Pressure before previous pressure (Pa)    P0: Target pressure (Pa)    Gi: Integral gain (fixed)    Gp: Proportional gain (fixed)    Gd: Differential gain (fixed)
In the above PID control method in which the valve opening operation amount ΔVV is calculated for each control cycle to control the valve opening degree, control is stable under a condition that the gain values in the PID control calculation formula are optimal. However, it may take long time to reach a target pressure. In addition, it may take extremely long time to reach the target pressure if there is a large difference from the optimal condition in terms of gas type, gas flow rate, gas dissociation state or target pressure level.
In other words, the above control method takes long control time and requires gain optimization for each condition. Besides, hunting often occurs with a butterfly throttle valve which shows a very nonlinear relation between valve opening and exhaust speed.